Not applicable.
Not applicable.
(1) Field of the Invention
This invention generally relates to protective metallic coatings for components exposed to oxidizing environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a method for removing a damaged substrate region beneath an environmental coating or a bond coat for a thermal barrier coating (TBC)
(2) Description of the Related Art
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to protect the surfaces of such components with an environmental coating, i.e., a coating resistant to oxidation and hot corrosion. Overlay coatings are widely used as environmental coatings, particularly those of the MCrAlX type in which M is nickel, cobalt and/or iron and X is a reactive element such as yttrium, hafnium, or another rare earth or reactive element. When used in combination with a ceramic top coat, or thermal barrier coating (TBC), environmental coatings are referred to as bond coats. During high temperature exposure in air, an MCrAlX coating forms a protective aluminum oxide (alumina) scale that inhibits oxidation of the coating and the underlying substrate.
During long hours at elevated temperatures typical for gas turbine engines, there is considerable interaction between an environmental coating and its underlying superalloy substrate. This interaction results in interdiffusion of alloying elements present in the substrate and the environmental coating. A consequence of this interdiffusion is that the portion of the substrate immediately beneath the environmental coating undergoes changes in elemental concentrations, leading to degraded mechanical properties. As such, the substrate region beneath a coating in which interdiffusion occurs may be referred to as a damaged substrate layer.
Though significant advances have been made with environmental coating materials and processes for forming such coatings, there is the inevitable requirement to remove these coatings under certain circumstances. For example, removal may be necessitated by erosion or thermal degradation of the coating, refurbishment of the component on which the coating is formed, or an in-process repair of the coating or a thermal barrier coating (if present) adhered to the component by the coating. For the purpose of rejuvenating and reapplying an environmental coating, it is preferable to also remove the damaged substrate layer. Current state-of-the-art repair methods typically entail removal of the ceramic TBC (if present), such as by grit blasting technique, followed by a chemical stripping technique to remove the environmental coating. Typical stripping methods entail removing a metallic coating by electrochemical reaction between an electrolyte and the coating, which dissolves the coating.
Chemical stripping methods are often tailored for the particular chemistry of the coating, and as a result cannot be readily employed to remove a damaged substrate layer beneath an environmental coating, because of the extreme difficulty of controlling the composition of an electrolyte so that it will remove the damaged substrate layer and not the underlying undamaged portion of the substrate. If an excessive amount of undamaged substrate is removed, the consequence may be scrappage of the component as a result of excessive thinning of the substrate. The typical presence of compositional gradients in a damaged substrate layer also increases the difficulty of controlling the rate of damaged material removal, and may result in incomplete removal of the damaged layer. Further complicating the above are the typically complex geometries of gas turbine engine components, with the result that the removal of a damaged substrate layer is extremely difficult and time consuming.
In view of the above, it can be appreciated that existing repair processes do not ensure the removal of a damaged substrate layer formed as a result of substrate interdiffusion with an environmental coating. Therefore, it would be desirable if a method were available for removing a damaged substrate layer, particularly without significantly removing or damaging the underlying undamaged substrate. Such a process would preferably ensure that the damaged substrate layer is completely removed with a high degree of accuracy and within acceptable tolerance limits.
The present invention generally provides a method of removing the damaged surface layer beneath a coating on a component, such as a superalloy turbine, combustor and augmenter component of a gas turbine engine. The method is intended for removing the damaged surface layer without excessive removal of undamaged portions of the underlying substrate, after which a new coating can be applied and the component rejuvenated. The present invention achieves these advantages with the use of an automated grinding method in combination with steps that enable the damaged substrate layer to be removed with a high degree of accuracy and reliably. A preferred grinding method is computer numerical controlled, which provides the added advantages of high repeatability and dimensional accuracy.
The process of this invention generally includes evaluating the component to assess the depth of the damaged substrate layer, followed by sensing a plurality of points over the outer surface of the component to determine a three-dimensional outer surface profile thereof. A three-dimensional grinding profile beneath the outer surface profile is then established based on the depth of the damaged substrate layer beneath the outer surface profile. The component is then ground along the grinding profile such that the damaged substrate layer is substantially removed without significantly removing the undamaged portion of the substrate beneath the damaged substrate layer. The resulting ground surface of the component is then sensed at locations corresponding to at least some of the plurality of points over the outer surface of the component to verify removal of the damaged substrate layer.
In view of the above, it can be appreciated that the present invention provides a processing methodology for removing a damaged substrate layer from a component protected by a coating, so that a new coating can be applied and the component rejuvenated. The invention makes use of an automated grinding operation and part-specific geometric data so that only a desired amount of material is removed from the component with a high degree of dimensional accuracy. According to a preferred aspect of the invention, components with complex shapes can be ground without compromising stringent dimensional tolerances, such as those required for advanced airfoil shapes. In addition, the grinding method of this invention can be automated to the extent that minimal operator intervention is required, and various complex shapes can be reproduced with very little manpower.
Other objects and advantages of this invention will be better appreciated from the following detailed description.